The present invention relates to methods and systems for measuring the local index of refraction in optical materials, and, for example, for measuring the local index of refraction in optical waveguide structures, and, more particularly, in one embodiment, to a method and system which uses a tuning fork to modulate the height of a probe in a near-field scanning optical microscope (NSOM), thereby allowing for measurement of the local index of refraction of optical materials and of integrated optics structures, including optical waveguide structures.
In the field of microscopy, there is a growing need for higher spatial resolution. In the past, spatial resolution in optical microscopy and spectroscopy has been limited by diffraction. This diffraction limit is essentially dependent on the wavelength of the employed radiation. To get even higher spatial resolution, one has to go to the near-field regime.
Near-field scanning optical microscopy (NSOM), sometimes referred to as a scanning near-field optical microscopy (SNOM), avoids the diffraction limit by operating in the near field (i.e., in a spatial range much less than the wavelength of interest). In NSOM, an optical probe is generally used as a subwavelength-sized radiation source to emit light or as a subwavelength-sized aperture to collect light. An instrument utilizing NSOM can produce high-resolution optical imaging, characterization, and surface modification.
A typical NSOM optical probe comprises a tapered optical fiber coated with a thin metallic layer to create the aforementioned aperture. In order to operate this probe in the near-field, the scanning device must be able to actuate the probe in a controlled manner at distances over the surfaces of interest that are within the nanometer range. Moreover, a feedback system is typically implemented to keep the probe-to-sample distance constant. Keeping this distance constant operates to prevent probe and/or sample damage, as well as to ensure proper interpretation of the optical results.
In addition to providing near-field optical information, NSOM also provide simultaneous topographical images using atomic force microscopy (AFM) techniques. Conventionally, NSOM topographical images are produced using shear force feedback. In this mode, the fiber is attached to an actuator that oscillates the fiber end (i.e., the probe tip) near its resonance frequency and generally parallel to the surface of the sample. The probe is held in near-field distance of the sample by a feedback system, with the feedback also providing a topographical image of the sample.
More particularly, during the oscillation, as the probe approaches the sample""s surface, forces between the tip and the sample result in the probe""s oscillation amplitude being damped and creates a phase shift. This damping is a function of the distance between the sample surface and the probe tip. Thus, the interaction between the probe tip and the sample surface is used to keep the probe-to-sample distance constant.
Conventional optical systems are not capable of producing images of a sample""s local indices of refraction. These and other methods average index of refraction measurements over part or all of the sample. Thus, there is a need to find methods and systems that measure local indices of refraction.
In one embodiment of the present invention, a method for determining a spatially local index of refraction in optical material is provided. Light, including a near-field intensity, is collected above a surface of the material. A probe is oscillated at a plurality of frequencies and in a substantially perpendicular manner relative to the surface of the material to detect the near-field intensity. A distance of the probe from the surface of the material is modulated. Based on a ratio of the near-field intensity of the light detected at the plurality of frequencies, the local index of refraction is determined.
In another embodiment of the present invention, a microscopy system is provided. The system includes a radiation source, a probe, an actuator, a modulator, and a processing module. The radiation source is configured to generate light in an optical material having a surface. The probe is configured to detect the near-field intensity of the light which is present just above the surface of the material. The actuator cooperates with the probe and is configured to oscillate the probe at a plurality of frequencies. The modulator cooperates with the probe and is configured to modulate a distance of the probe from the surface of the material. The processing module communicates with the probe and is configured to determine a local index of refraction of the material based on a ratio of the near-field intensity of the light detected at the plurality of frequencies.
Still other advantages and novel features of the present invention will become apparent to those skilled in the art from the following detailed description, which is simply by way of illustration various modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different obvious aspects, all without departing from the invention. Accordingly, the drawings and descriptions are illustrative in nature and not restrictive.